Abstract: Arm support for a chair comprising an upright stand (2) fixed to the chair and having an upper end which supports the arm support (1), said arm support (1) being movable on a substantially horizontal plane and possibly turnable around a vertical axis. The arm support (1) comprises a first guide plate (9) provided with guide openings (12) arranged on the rims and running along the longitudinal direction of the arm support, in which sliders (15) coupling geometrically the guide openings (12) are present, said sliders (15) being tightly connected to a second guide plate (14), the length of the second guide plate (14) along the shift direction being smaller than the length of the first guide plate and the second guide plate (14).